This is the answer I get when I ask: when am I done with my book? When it’s as good as you can possibly get it.
Well, never then. I can edit forever.
So I need to figure out some other kind of guideline. Maybe it’s done when I stop carrying my idea notebook with me everywhere I go. Or when I stop coming up with new ideas in the middle of work or during my commute. Or when it’s not the first thing I think of when I wake up in the morning.
Maybe it’s finished after I’ve done two more read-thrus after I said I was doing my last read-thru. Maybe after a year of beginning each weekly writing workshop with the disclaimer to my fellow writers: Sorry it’s more nun stuff. One year after the writing residency. Two years after I finished the first draft. Three years after I began.
Maybe it’s done when it stops feeling vital. When it stops being the answer to the question: how are you? When I start having dreams at night about other books I might write.
Anne Foster 
Yeah, this is a hard thing to pinpoint. At a certain stage, all your editing just makes the story different-er, and not necessarily better. This is something we all have to learn on our own, am I right? Anyway, wishing you the best on your journey!
I like that observation! Different but not necessarily better–I think I’m getting there. Good luck to you too!